1. Field of the Invention
The invention relates to thread handling systems for lockstitch sewing machines.
2. Description of the Prior Art
Conventional thread handling systems in lockstitch sewing machines include a tensioning device with a check spring, and a takeup member which pulls thread through the tensioning device from a spool and sets a stitch during an up stroke, and which supplies thread for use by a needle and looptaker during a down stroke. Various irregularities often occur in the stitches and material sewn with such systems due to the action of the check spring, the presence of excessive slack in the needle thread when tension is required to force the thread into the throat of the looptaker, and the inability of the tensioning device to supply thread at a light tension in response to a demand by the looptaker. The irregularities include a varying degree of tightness in first stitches, the occurrence of haloing characterized by the formation of slack loops in stitches as described for example in U.S. Pat. No. 4,095,539, the puckering of material, and skipped stitches. Furthermore, conventional thread handling systems severely limit the thickness of materials which can be satisfactorily sewn on a machine. The conventional systems also prevent the work feeding operation and needle penetration of the work to be timed so as to satisfactorily meet stitch forming requirement while avoiding damage to the needle.